Vacuum-tube discharge device



Dec. 10, 1929. A. MUTSCHELLER VACUUM TUBE DISCHARGE DEVICE Filed May 13.1922 mum: l

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A TTORNE Y l at'ented Dec. 10, 1929 UNi'rn ARTHUR MU'ISGI-IELLER, 015NEW YORK, N. Y.

VACUUM-TUBE DIECHARGE DEVICE Application filed May 13,

My invention relates to vacuum tube discharge devices: that is. tocathodes and parts immediately associated therewith constructed andarranged with a view to controlling electrical discharges through vacuumtubes More particularly stated my invention comprehends a vacuum tubedischarge device for use in a highly evacuated tube and is suitable, forinstance, for use in l d-ray work.

At one time it was generally believed in this art that electricity isincapable of traversing space in a very high vacuum, but it wasafterward shown that by using a certain specific form of negativeelectrode and a sufficiently high potential such as that from anordinary Euhmlrorff coil, a discharge could be forced through the tube.It was also observed that large quantities of X-rays were produced ifthe electrodes were near enough together and the cathode wasprovidedwith a point sufliciently sharp. lt was further observed thatthe better the vacuum the greater theintensity of X-rays produced andsent out.

These observations therefore brought out mainly the fact that the pointof the cathode should be very sharp; that is, that the angle between thelongitudinal axis of the cathode and the adjacent sloping side of thecathode must be very small. Otherwise a discharge will not take place,at least not unless a very high voltage is applied. Such being the case,it follows that the control of the discharge depends mainly upon thesharpness of the pointor edge of the negative electrode.

Then there is known in this art an X-ray tube of the so-called hotcathode or Coolidge type. This tube has a cathode to be heated which isin the form of a disc or flat coil and the discharge from it atsaturation is inde pendent of the voltage applied to its electrodes.This and all other known hot cathode type tubes therefore have asaturation condition or rigid and inelastic characteristics.

1 have made the discovery that if the negative electrode is cone shapedor knife shaped, and the angle of the point of the cone is so large (orin other words if the point of the cone is so blunt) that a largedischarge variable with the voltage and at a 1922. Serial 'NO. 560,545.

normal voltage does not take place, a discharge of any desired magnitudecan be obtained if the sharp edge of the cathode is heated. Also, thatthe degree of heat required in such a pointed cathode used for thispurpose, in order to obtain a discharge of predetermined magnitude, isless than would be required for an equivalent discharge by use of aheated negative electrode, the end of which is flat, round or helical.The same principle holds good if the cathode have a sharp edge like thatof a knife blade, instead of a point. a

By taking advantage of the foregoing observations I have produced animproved vacuum tube discharge device possessing various advantages overother discharge devices heretofore known, and particularly over thoseusing so-called thermal-electronic or thermo-ionic principles.

Among the advantages of my device are the following: a

I. It enables much larger discharges to be obtained than would bepracticable with a cathode in which the cathode point, no matter howsmall or how fine it may be, is cold.

111. It enables the effective temperature of the cathode, with its sharppoint heated, to be maintained considerably lower than would be requiredfor equal discharges from a U cathode the end of which is flat, round orhelical.

Ill. It dispenses with the necessity for an electrostatic focusingdevice for many purposes, although for other purposes one may be used.

TV. The discharge produced is dependent upon the voltage applied to theelectrodes, so that my device in its action is much less irregular thana device of the kind in which the discharge is independent of thevoltage; Therefore, changes or variations in either both the cathodeheating or the discharge current are less disturbing upon the generalfunctioning of the elastic discharge device of my invention. a

V. By using cathodes of varying degrees of sharpness, it is practicable,with a given cathode temperature, to obtain discharges varyingconsiderably in volume and thus of developing a great variety ofcurrent-voltage characteristic curves.

Reference is made to the accompanying drawing forming a part of thisspecification, and in which like reference characters indicate likeparts throughout all of the figures.

Figurel is a side view, partly in section and partly diagrammatic,showing one form of my device as incorporated in an X-ray tube.

Figure 2 is a fragmentary section showing the X-ray tube cathode andparts immediately associated therewith.

Figure 3 is an end elevation of the cathode and parts immediatelyassociated therewith ap earingin Figure 2.

igure 4. is a section showing a form of cathode made in accordance withmy invention, but different from the form appearing in Figures 1 to 3inclusive.

Figure 5 is an end elevation of the cathode and parts immediatelyassociated therewith shown in Figure 4.

Figure 6 is a fragmentary section showing an improved anode adapted foruse with either form of my improved cathode.

, An X-ray tube appears at 7, and i provided with an anode neck 8 and acathode neck 9,.these parts being of the usual or any preferredconstruction. The anode is shown at -10,and is made'of copper or othersuitable metal. It carries a target face 11, made of tungsten and it maybe fiat or provided wi h a concavity 12 of the form shown. Thisconcavity has a portion 12 with a rather sharp curvature, and-also has aportion 12 which has relatively little curvature and is so disposed'thatX-rays, originating in the portion 12, have an unobstructed pathdirectly downward from their point of origin. This ar rangementfacilitates the equal distribution of the X-rays downwardly, and has atendency to render them equally dense per unit of area exposed.

Mounted within the cathode neck 9 are two cathode members 13, 14,provided respectively with portions 15, and 16 of reduced diameter,these portions being tapered of? thin and meeting at a sharp point 17,so that the cross section of the reduced portions 15 and 16 is verysmall in immediate proximity to the point 17. The superficial angle madeby the meeting of the portions 15 and 16 is acute,

preferably 45 degrees or less, as may be understood from Figures 1 and2.

A metallic plate 18, made preferably of aluminum, is provided with asupporting flange 19, the latter being secured to the stem 14 by meansof a screw 20 or other appropriate fastening. The portion 16 extendsthrough a hole in the plate 18, into which it fits tightly. The portion15 extends through a hole 21 in the plate, this hole being of adiametersomewhat larger than that of the portion 15, so as to leave a clearanceall around it, as may be understood from Figures 2 and 3.

A battery is shown at 22, and is used for supplying current to heat thepoint 17, as hereinafter described. A wire 23 is connected with thebattery 22, and leads therefrom to a variable resistance 24. From thisvariable resistance a wire 25 leads to the stem 13. The stem 14 isconnected with a wire 26, leading to the battery 22. A wire 27 isconnected with the wire 26, and constitutes one of the main leads of theX-ray tube, the other of these leads being connected with the anode 10and being shown at 28.

The battery circuit, used for heating the point 17 may be traced asfollows: battery 22, wire 23, variable resistance 24, wire 25, stem 13,point 17, stem 14 and wire 26, back to battery 22.

In the form of cathode shown in Figures 4 and 5, I provide a metallictube 29 carrying, a conical member 30 of metal. This member has a slot31 extending through it in the direction of its axis. Located within theslot 31 are a pair of metallic conductors 32 and 33.

Supporting members 34, made of insulating material extend inwardly fromthe conical member 30 and brace the same relatively to the stems 32 and33, and a metallic connection 35 is made from the conductor 33 to theconical member 30. A fine wire 36, or conducting filament, extends fromone of these stems to the other, and is provided with a portion 37, bentto an acute angle and extending out from the slot 31 so that the point37 is both the apex of the conical member 34 and the heated sharp pointof the cathode.

The conical member 30, though preferably of aluminum, may be made of anymetal known in this art as non-sputtering metal. The wire 37 may be madeof tungsten. platinum, molybdenum, tantalum. or am' other metalsufiiciently refractory. This is also true of the conductors 32 and 33,and especially the portions thereof leading out to the point 37. Ifdesired, instead of the member 36 being a wire, it may be a filament ofany other material suitable for use in a highly evacuated vessel andcapable of withstanding the effects of the heating and of thedischarges.

The plate 18, arranged as indicated in Figures 2 and 3, and also theform of cathode shown in Figures 4 and 5, appear to be of materialassistance in facilitating the main discharge, and also in focusing anddirecting the same.

The point 17 or 37 as the case may be, is located at any suitabledistance from the anode 10, depending upon the method employed forfocusing the discharge or the shape and size of the focusing shieldemployed for the purpose of obtaining the desired small focal area, butis preferably spaced therefrom to such an extent that the point is notless than two millimeters nor for ordinary purposes more thantenmillimeters from the concavity 12 shown in Figure 6. This distance maybe greater in X-ray tubes which have flat targets or which are used withthe higher voltages employed for deep therapy.

l Vith the sharp point 17 or 37 properly shaped, and heated as abovedescribed, a main discharge of predetermined proportions may be broughtabout by the use of a temperature or emitting area considerably lower orby an emitting area much smaller than. is ordinarily required to producea like discharge by the use of a heated cathode having a helical orspiral form such, for instance, as has heretofore been used in X-raytubes of the Lilienfeld type and of the Coolidge type.

The two specific forms of cathode above described are merely typical ofmy invention, as I contemplate using a cathode having any suitable formin which there is a sharp por tion of any desired shape and heated inany of the ways known in the art. As may be understood from Figures 3and 5, the point 17 or the point 37 is not only a point serving the apexof a cone, but is also a sharp edge somewhat resembling that of a knifeblade.

It will be noted that, owing to the form of the tungsten target face 11of the anode 10, the X-rays are produced within the concavity 12 and inradiating downwardly from this point of origin they are not obstructed,and are therefore substantially uniform per unit of area to which theyare exposed.

The operation of my device may be readily understood, in the light ofthe prior art, from the foregoing description.

The tube is connected up with a suitable source of current ofhighpotential, and aside from the distinctions herein pointed out, isused in substantially the same manner as any other X-ray tube of highpower.

The fundamental difference between my improved vacuum tube and those ofthe prior art may be illustrated as follows: By taking readings of themilliamperes of cur-- rents passing through the tube in the maindischarge, and of the voltage across the main terminals of the tube, andplotting the logarithms of the values thus observed, it will be foundthat these values, in the case of gas tubes, are generally located alonga straight line the slope of which is very close to an angle with thelog V line the tangent of which has a value very close to 2. Thus itscharacteristic may be expressed approximately as 100 712.60 2 log V+ 0.

Plotted in the same manner, the readings obtained from a tube of theCoolidge type operated above its saturation point, are represented by astraight line, the angle of which with the log V line is ordinarily ofzero value; that is, parallel with the log V line. This indicates aninflexible system, independent of the voltage applied under operatingconditions. When, however, the data is plotted in the same way with myimproved tube, there is obtained a straight line the slope of which maybe any value from zero to any intermediate value up to the full value oftwo, or even a high value. This means that beginning with a very bluntcathode corresponding to the kind generally employed in the Coolidgetube, and making the cathode point successively sharper and sharper, Iobtain any desired curve slope, and therefore greater adaptability thanis ordinarily obtainable, even by use of a gas tube.

It follows, therefore, that my. improved X- ray tube possesses theso-called elastic characteristics of the gas tube instead of thesocalled inflexibility of the Coolidge tube; and in addition, my tubepossesses further adaptability in that by varying the superficial angleof the heated point, or in other words by rendering the cathode pointblunt or sharp, the operating characteristics may be readily variedpredetermined and standardized.

While my device is well adapted for use as an X-ray tube as abovedescribed, it may be employed to advantage in vacuum tubes of all kinds,such as rectifiers, amplifiers, audions, pliotrons, kenotrons, andgenerally speaking in all forms of valve tubes.

I do not limit myself to the precise structure shown, as variations maybe made therefrom without departing from my invention, the scope ofwhich is commensurate with my claims.

I claim:

1. In a discharge device the combination with a highly evacuated vesselof an anode with great active area and a filamentary cathode havingportions converging to a point whose cross sectional area is a minimumcompared to said converging portions.

2. In a discharge device an anode and a filamentary cathode havingportions converging to a point Whose cross sectional area is a minimumcompared to said converging portions.

3. An X-ray tube as claimed in claim 1 designed to carry a current whichis equal to the nth power of the voltage applied where n is a constantdepending upon the apex angle of the filamentary cathode.

4;. In a discharge device the combination with a highly evacuated vesselof an anode with a great active area and a filamentary cathode havingportions extending to a point where the heating is a maximum compared tosaid extending portions, the discharge device being designed to carry acurrent which is equal to the nth power of the voltage applied where nis a constant depending upon the apex angle of the filamentary cathode.

ARTHUR MUTSCI-IELLER.

